Thin film battery cells are typically heat sealed in soft aluminum/plastic laminate foil pouches. U.S. Pat. No. 6,284,406 discloses a thin film cell battery sealed by a sheet of flexible laminate material having at least one metal foil layer and a layer of an adhesive-like polymeric material. Such a sheet would provide multiple layers, but may not be thick enough to provide sufficient resistance to the undesired diffusion of chemical species in applications such as lithium ion batteries. The sheet provides a single seal, leaving it prone to many problems.
U.S. Pat. No. 5,654,084 discloses an enhanced protective coating to prevent interaction between constituents of the environment and devices that can be damaged by those constituents. The protective coating comprises alternating layers of an organic coating (such as ParyleneSM) as a diffusion barrier, and a ceramic or metal coating (such as aluminum) as a physical barrier. For thin film battery cells, where diffusion is the primary concern rather than physical breach, the inclusion of a ceramic or metal physical barrier adds unnecessary weight, expense and complexity. Again, laminate foils alone do not provide sufficient protection.
The problems single foil seals are most prone to are:
1. They exhibit poor diffusive resistance. Electrochemical cells, and in particular lithium ion cells, react in an undesirable manner with atmospheric components such as water vapour, oxygen, nitrogen and carbon dioxide, damaging the cell components. In cells comprising a liquid electrolyte, the electrolyte may also diffuse out of the cell.
2. They are generally fragile and are easily nicked or torn. The seal may be breached through mechanical or thermal shocks. This can cause electrolyte leakage, which impairs battery performance and introduces dangerous high voltage connections.
3. The aluminum in the foil may cause voltage leakages. During cell packaging, the foil is generally freshly cut. Not enough time elapses for the aluminum to develop an electrically insulating oxide layer, so conductive pure aluminum is exposed. This may result in voltage leakage or short circuits, particularly at the terminals.
Thus it is desirable to package a cell in such a manner as to minimize contact between the cell components and the external environment.
Most foil-sealed pouch cells are further packaged into prismatic metal enclosures which are welded shut. This addresses many of the problems listed, but is expensive and requires precision tooling. These containers corrode over time due to voltage ingress, eventually rendering them useless.
United States Patent Application Publication No. 2002/0071989 discloses a thin film battery layered with a protective dielectric coating and then covered by a sealing layer comprising epoxy. This results in a cell with a plurality of protective layers, but only one seal. This solitary epoxy seal provides only limited protection against diffusion. With respect to the problems listed above, this arrangement is not a significant improvement over laminate foils. Furthermore, improper epoxy curing processes can lead to degradation of some metals in contact with said epoxy.
U.S. Pat. No. 6,924,060 discloses a sealed battery module wherein a sheet is folded over a row of cells to form a seal. The sheet possesses inserts and elastic pressing members, such that when applied will form an additional seal comprising the inserts and the current collectors. In this manner, the electrolyte seal for the individual battery cases is secured by a double seal, while the rest of the cell has only a single seal. Such a method of sealing cells within a battery module by the use of sheets with inserts applies only to battery modules comprising a plurality of cells arranged in a line, and does not allow for the packaging of individual cells nor variance in geometrical arrangement.
Another challenging aspect of thin film battery cells, as for example lithium ion batteries found in most portable electronic devices, electric vehicles, and other applications, relates to thermal management. High temperatures increase the rate of exothermic intracellular reactions, which increase the rate of heat generation, leading to even higher temperatures. This situation is know as thermal runaway and is damaging to the cell, sometimes leading to combustion or explosion. In addition to destroying the battery, this also poses a safety risk. It should therefore be desirable to include within a lithium ion battery means of dissipating heat to the external environment.
One of the advantages of flat-geometry, pouch type cells is the greater surface area to volume ratio as compared to cylindrical cells, allowing for better heat dissipation. U.S. Pat. No. 6,703,160 proposes to increase the effective surface area of the cell by increasing the outer surface roughness of the battery by rubbing with an abrasive or by indenting the surface to increase the effective surface area.
Another strategy has been to improve convective heat transfer by increasing fluid flow around the cells in a battery pack and including baffles or heat fins. U.S. Pat. Nos. 7,867,663 and 7,896,063 employ these concepts in their disclosures.
Another option is to improve thermal radiation from a cell to its surroundings. The best means of accomplishing this is to increase the effective emissivity of the cell. U.S. Pat. No. 6,703,160 suggests the outer surface of a battery be formed of high emissivity material and that a high emissivity label be applied. High emissivity paints and coatings are also abundant, though they are primarily used in non-battery applications.
A further concern is the temperature distribution within a cell or battery pack. Due to cell geometry, combined with the electrical leads which function as heat sinks, some areas generally exhibit higher temperatures during operation. These higher temperature areas degrade more quickly and impair performance compared to the cooler areas of the cell.
All of the above applications and patents are incorporated herein by reference; but none of these references is admitted to be prior art with respect to the present invention by its mention in the background.